Classically, information can be encoded digitally by using a collection of systems which can each be placed in one of two distinct states, labelled as zeros and ones. Each system carries one “bit” of information. On the other hand quantum mechanics has systems that can be placed in states, called “superpositions”, that exist simultaneously as zeros and ones. For example information could be encoded with single photons by labelling a horizontally polarized photon, |H>, as a one and a vertically polarized photon, |V>, as a zero. But a photon can also be in a superposition of these two states such as 1/√{square root over (2)}(|H>+|V>). Such a state carries one “qubit” of quantum information [1]. Furthermore quantum systems can be “entangled”. Entanglement refers to correlation between the measured properties of sub-systems that cannot be explained in terms of classical correlation.
These different properties of quantum information make new technologies possible, in particular “quantum cryptography” [2] which uses quantum information to distribute cryptographic keys with near perfect security and “quantum computing” [3] which utilizes the increased processing power of quantum information to dramatically reduce the time required for certain calculations. Whilst photons are easily manipulated at the single qubit level, long distance quantum cryptography and all quantum computing requires two qubit gates. Two qubit gates are not easily implemented in optics.
A key two qubit gate is the Controlled Not (CNOT) gate. It operates in the following way: One qubit is the “control” and the other qubit is the “target”. The control qubit emerges from the gate in the same logical state as it entered. If the control qubit is in the logical zero state then the target qubit emerges with the same state as it entered. If the control qubit is in the logical one state then the target qubit emerges with the opposite state to that with which it entered the gate. If the control qubit is in a superposition state then the control and target qubits become entangled.